1. Field of the Invention
The present invention relates generally to the casting of aluminum alloys, in particular the casting of alloys containing magnesium and/or lithium, sensitive to oxidation.
2. Description of Related Art
The oxidation of aluminum alloys in a liquid state has detrimental consequences on the casting process. In furnaces and transfer troughs the oxidation of metal initially results in a net metal loss, called loss on ignition. In addition, during casting, too great an oxidation of the molten metal generates surface defects on the ingot cast which have a detrimental effect when the products are used. These problems are particularly marked in alloys containing magnesium and/or lithium.
A main defect is the vertical fold which is caused by crumpling of the oxide skin on the surface of the sump. In certain cases, and in particular when casting 7xxx alloys, this problem is particularly great because the folds, especially when they are long and deep, easily cause surface cracks. Folds and cracks must generally be eliminated before transforming the ingots obtained during casting. The defects may, for example, be eliminated by machining, which may be a very economically unsatisfactory solution, in terms of both the cost of the operation and the significant metal loss which occurs as a result. In certain cases, the presence of cracking makes the ingot unusable and it has to be remelted.
It has long been known that adding certain elements makes it possible to limit oxidation and to improve the surface quality. In 1943, U.S. Pat. No. 2,336,512 described the addition of very small quantities of beryllium to aluminum alloys containing magnesium in order to limit oxidation of the molten metal surface. International application WO 02/30822 described the substitution of beryllium by calcium with the same aim of limiting oxidation.
But the use of additives may cause other problems. Beryllium, for example, is to some extent toxic which has led to its removal from aluminum alloys used for food packaging. Calcium may lead to edge cracking during hot rolling.
It has also been proposed to protect the surface of the molten metal by means of various devices. U.S. Pat. No. 4,582,118 proposes using a non-reactive and non-combustible atmosphere, such as for example, an atmosphere of argon, helium, neon, krypton, nitrogen or carbon dioxide, for casting aluminum-lithium alloys. But such processes are very expensive to use.
Patent application EP 0 109.170 A1 describes the use of a baffle on the edge of the casting device to sweep the molten metal surface with an inert gas (usually nitrogen and/or argon with or without chlorine or another halogen). But these gases are tricky to use and significantly increase the cost of operations.
The use of carbon dioxide or combustion gas to limit oxidation is also known by C.N. Cochran, D. L. Belitskus and D. L. Kinosz, Metallurgical Transactions B, Volume 8B, 1977, pages 323-331. Patent application EP 1 964 628 A1 describes a method for producing aluminum ingots in which at least one stage of the process is carried out in an atmosphere containing a fluorinated gas. However, fluorinated gases are tricky to use and carry large safety risks.
U.S. Pat. No. 5,415,220 describes the use of molten salts of lithium chloride and potassium chloride to protect the surface of aluminum-lithium alloys during casting. But the drawback to using molten salts is the risk of contamination of the molten metal with impurities, as well as the difficulty of using them.
U.S. Pat. No. 7,267,158 describes the forced addition of a wet gas, containing more than 0.005 kg/m3 water on the surface of the molten metal in order to improve the surface quality of the cast ingots. This process has, however, the disadvantage of bringing the water vapor and liquid aluminum into contact with each other, in spite of the dangers of explosion caused by contact between water and liquid aluminum. In addition, it is known from application EP 0 216 393 A1 that dry air can be used in a treatment ladle for liquid aluminum to prevent hydrogen from penetrating into the molten metal when a treatment gas is injected into the molten metal and causes the oxide coating protecting its surface to burst.
A problem was to find a casting process suitable for most oxidable aluminum alloys, in particular aluminum alloys containing magnesium and/or lithium, which does not have these disadvantages and makes it possible to obtain cast ingots that are free or virtually free from surface defects and pollution, in as complete safety as possible.